The Rules of Contagion

If we want to predict a person’s risk of infection, it’s not enough to
measure how many contacts they have. We also need to think about
their contacts’ contacts, and their contacts’ contacts. A person with
seemingly few interactions might be just a couple of steps away from
a high transmission environment like a school. A few years ago, my
colleagues and I looked at social contacts and infections during the
2009 flu pandemic in Hong Kong.[18] We found that it was the high
number of social contacts among children that drove the pandemic,
with a drop in contacts and infection after childhood. But there was a
subsequent increase in risk when people reached parenthood age.
As any teacher or parent will know, interactions with children means
an increased risk of infection. In the US, people without children in
their house typically spend a few weeks of the year infected with
viruses; people with one child have an infection for about a third of
the year; and those with two children will on average carry viruses
more often than not.[19]
As well as driving transmission in communities, social interactions
can also transport infections to other locations. In the early stages of
the 2009 flu pandemic, the virus didn’t spread according to the as-
the-crow-flies distance between countries. When the outbreak started
in Mexico in March, it quickly reached faraway places like China, but
took longer to appear in nearby countries such as Barbados. The
reason? If we define ‘near’ and ‘far’ in terms of locations on a map,
we’re using the wrong notion of distance. Infections are spread by
people, and there are more major flight routes linking Mexico and
China – such as those via London – than those connecting Mexico
with places like Barbados. China might be far away for a crow, but it’s
relatively close for a human. It turns out that the spread of flu in 2009
is much easier to explain if we instead define distances according to
airline passenger flows. And not just flu: followed similar airline
routes when it emerged in China in 2003, arriving in countries like the
Republic of Ireland and Canada before Thailand and South Korea.
[20]
Once the 2009 flu pandemic arrived in a country, however, long
travel distance seemed to be less important for transmission. In the
US, the virus spread like a ripple, gradually travelling from the